What are layers?
Figma works with layers. Every object is a layer, but if you group objects, the group has its own layer as well.
This is a hierarchical system, so the highest layer in the layerlist will be shown first. And everything below it will only be shown when there's nothing 'above' it. If you can see an object, but it's in the layer list. It's probably 'behind' another layer.
Layers can be found in the layers pane on the left. You can drag the layers in this pane to adjust the order of the layers. You can also rename layers by double-clicking on them.
Another way to rearrange the order is by right-clicking an object and select "Bring to front" (])or "Send to back" (]).